Industrialization has raised the standard of living throughout most of the world. However, manufacture of material goods results in creation of chemical waste byproducts that have been dumped, stored on-site or moved to storage dumps. The storage of heating fuel and other hygroscopic hydrocarbons in underground or surface tanks made of iron over many years has resulted in deterioration and rupture of the tanks. The hydrocarbon liquids can then percolate into the adjacent soil creating a hazardous mixture or can contaminate potable water supplies underlying the spill. The number of sites requiring remediation throughout the world is staggering.
Though the severity and seriousness of the soil contamination problem has been recognized and efforts to clean up the sites has started, there is a continuing accumulation of hazardous materials from current manufacturing activities and there are fewer and fewer sites available which will accept and store such wastes. Unless the contaminated soil can be treated on-site, transportation of large amounts of contaminated soil over long distances may be required and can add a substantial cost to the manufacturing process and product. The generator of the waste is also required to pay annual fees to store the waste.
On-site remediation can cost as much as $200 to $400 per ton of soil and may only result in a remediation of a portion of the soil. Depending on the soil type and grain size distribution, the contaminants may concentrate into a fraction of the soil. This fraction may still require transportation and storage at a designated hazardous waste storage facility.
Incineration can be an appropriate on-site means to remediate contaminated soil. However, fuel is expensive and the stack gases often contain undesirable air pollutants. In-situ soil cleaning with solvents has also been proposed as a means of remediation. However, solvents are expensive, may be hazardous or toxic, are often flammable and the dissolved mixture can migrate into underground water supplies thereby creating a worse problem.
Soil washing is another potential on-site remediation technology. Soil washing can be defined as the ex-situ treatment of contaminated soil using water as the primary solvent. The cleaned fraction is returned to the excavated site. Oversize materials are mechanically removed from the soil and may be treated by spray washing to remove contaminants.
Soil washing has been practiced in Europe since the mid1980's and since 1990 it has been approved in the United States for remediation at 17 Superfund sites. The technology is most effective in remediating coarse grained sands or gravels contaminated with organic or inorganic compounds.
The fine grain clay or silt fraction below about 70 microns presents difficult problems in removing the contaminants by traditional soil washing techniques. Over time, as a result of migration, weathering and degradation, soil contaminants, having a greater affinity for the fine-grained materials, will tend to accumulate and concentrate on the fine grain particles. The physical characteristics of the fine grain particles result in greater adhesion of contaminants than on the coarse grain fractions. The fine grain particles have greater surface area and greater adsorptive binding forces than do larger particles. The surfaces of clay particles can be charged which contributes to adsorption and also to dispersion of the particles as colloidal suspensions in the liquid phase. The fine particle fraction is difficult to treat and to separate from the liquid phase.
Due to the difficulty in removing the contaminants from the fine fraction, soil washing is used in conjunction with other remediation operations such as incineration or soil washing is used to concentrate the contaminants in the fine fraction which is then transported to a licensed storage facility. The volume reduction of the contaminated soil produced by traditional soil washing does therefore provide a cost benefit by substantially contributing to reduction in the volume of waste. The traditional process can concentrate 70-90% of the non-volatile organic and heavy metal residual products into the fine fraction representing 5-40% of the original soil volume. Reduction in volume can itself contribute to cost effectiveness. However, traditional soil washing does not clean all of the soil. The fine fraction, if still contaminated, must be stored permanently or until a feasible remediation technology is developed.
There have been attempts to augment the removal of contaminants from soil by agitation to provide abrasive scouring and/or scrubbing action. Surfactants can be employed to increase mobility of the washing fluid by reducing surface tension and to enhance release of the hydrophobic organic contaminant from the surface of the soil particles by reducing interfacial tension (IFT). Though recovery of contaminants is improved, a substantial amount of organic contaminant typically remains with the fine fraction.
Another disadvantage of traditional soil washing is the movement of contaminant into the wash water and the necessity and expense of treating and disposal of the wash water and the need to constantly add substantial amounts of make up water.
Another method of cleaning contaminated soil is soil flushing. Soil flushing can be practiced ex situ at the surface or in situ. A liquid is applied to a column of soil percolates downward and mobilizes the contaminant and transports it downward to a collection zone where the mobilized contaminants collect. The solution of contaminants is purged to the surface from this zone. The leach liquid can flow by gravity or can be placed under pressure to pump it through the contaminated zone. In situ delivery can include surface flooding, ponding, spraying, filtering and subsurface infiltration beds and galleries.
Soil flushing accelerates the recovery of contaminant from the soil. The flushing fluid can be water, enhanced water or gas to accelerate dissolution reactions such as sorption, acid-base solutions, precipilation, oxidationreduction, ion pairing or complexation or biodegradation reactions. Furthermore, soil flushing can accelerate transport of contaminants by advection, dispersion, diffusion or depletion by volatilization or dissolution.
Soil flushing has an especially useful application for cleaning soil beneath structures and especially soil contaminated with chlorinated hydrocarbons. No excavation or surface access is required. There also are no soil replacement or disposal costs. Exposure to workers is less than in ex situ methods.
Removal efficiencies appear related to site hydrogeology. Channelling or finger flow can occur which results in nonuniform removal of the contaminant throughout the zone. The relationship between capillary pressure, water content and permeability is not generally understood. Furthermore, the contaminant appears to have stronger affinity for smaller particles. For these reasons, it has been recommended that soil flushing applications be limited to sites with sandy soil, organic carbon content &lt;2% clay and silt fines &gt;15% bind having a high hydraulic conductivity (&gt;10.sup.-4 cm/sec).
It has been noted that the rate of percolation of a flushing liquid through a zone of soil is reduced on successive flushing. It is believed that the reduction in permeability is due to plugging of pores with fine grain material, large surfactant molecules, bacterial buildup or swelling of the soil. The size of the surfactant molecules appears to be the predominant factor since flooding with water without surfactants does not result in a significant reduction in permeability.
The residual hydrocarbon concentration is controlled by the water solubility of hydrocarbon contaminants, the interfacial tension (IFT) between the contaminant and water and soil and the relative permeability of the contaminant and water. The very low solubility of most hydrocarbons in water restricts the use of water flushing. However, proper surfactants can effectively reduce the IFT and can enhance hydrocarbon recovery by suspending the hydrocarbon in an emulsion phase.